Existing Technology
In the conventional proximity switch using high frequency oscillation, oscillation of a fixed frequency is produced by an oscillator circuit which contains an oscillator coil. When a metallic object approaches, eddy current loss causes a change in the conductance of a detector coil. The signal level in the oscillator circuit will decrease, or oscillation will cease entirely. By detecting this change in the signal level, one can detect the proximity of an object. The sensitivity of this detection scheme is good with respect to magnetic metals such as iron, which generate a substantial eddy current loss; but it is lower for non-magnetic metals such as aluminum, which cause only a slight eddy current loss. There has thus been a demand for a proximity switch capable of detecting any metal. Such a switch would have a fixed sensitivity regardless of the composition of the object to be detected, that is to say, regardless of what type of metal the object was composed of.
A proximity switch which can detect any metal is described in Japanese Patent Kokai-Koho 55-72329. The oscillator circuit has a feedback circuit which is tuned to a frequency higher than that of the resonant circuit, producing a high impedance. A magnetic metal is detected using eddy current loss according to the commonly applied principle. A non-magnetic metal is detected via a change in oscillating frequency, which is obtained by detecting the decrease in feedback signal strength with a filter.
Another example is given in Japanese Utility Model Koukoku-Koho 53-30672. Two coils are used, one functioning in a detector circuit and the other in a reference circuit. A magnetic object is detected when oscillation ceases as a result of the drop in the Q of the detector circuit. A non-magnetic object is detected when oscillation ceases because the change in impedance causes there to be a large difference between the oscillating frequency of the detector circuit and the resonant frequency of the reference circuit.
Yet another type of proximity switch is described in Japanese Patent Koukoku-Koho 50-14749. This switch features a parallel resonant circuit with a series resonant circuit magnetically coupled to it. When viewed from the parallel circuit side, the impedance characteristics are double-humped, and the attenuation characteristics of the output are consequently fixed regardless of the type of metal being detected.
These proximity switches, which are capable of detecting any type of metal, all require a number of coils, so it is difficult to downsize them. In addition, if the characteristics of these coils do not allow them to remain in a fixed relationship with each other, it becomes impossible to achieve stable output characteristics. In the real world, there are great variations in the characteristics of components, and inductance and other parameters vary with temperature, resulting in poor stability. The necessity of using more than one coil complicates the design and makes the switch more difficult to adjust. It also makes it more difficult to lower the cost.